1. Field of the Invention
The present invention relates to a zoom lens system for a single lens reflex camera, and in particular, to a macro zoom lens system which enables photographing an object at an extremely short distance.
2. Description of the Related Art
In the prior art, most of zoom lens systems with capability of short-distance photography are arranged to perform short-distance photographing at only one of the short focal length extremity and the long focal length extremity. Furthermore, in these zoom lens systems, zooming cannot be performed during short-distance photographing, which is inconvenient for a photographer from the viewpoint of operations for the camera. On the other hand, U.S. Pat. Nos. 5,528,423 and 5,734,508 have disclosed zoom lens systems with capability of short-distance photography over the entire focal length range. However, when short-distance photographing is being performed, the object distance, i.e., the first lens surface of the most object-side lens element to the object, becomes too short, thereby, the maximum image magnification inevitably becomes smaller, such as 0.25 to 0.3. Therefore a requirement for photographing an object larger is not satisfied.
In recent years, zoom lens systems with capability of short-distance photography over the entire focal length range and with the maximum image magnification of more than xc2xd have been disclosed in U.S. Pat. Nos. 5,920,435, 5,717,527 and 5,793,531. However, these zoom lens systems are the front-lens-group advancing type lens system in which the most object-side lens group which is the largest in size and the heaviest in weight is arranged to be advanced. Therefore requirements for miniaturization and weight reduction are not satisfied.
It is an object of the present invention to provide a macro zoom lens system which enables (i) short-distance photography over the entire focal length range, i.e., a full-range macro photography and (ii) miniaturization and weight reduction, and (iii) has a sufficient object distance even when the maximum image magnification is more than xc2xd.
In order to achieve the above-mentioned object, there is provided a macro zoom lens system including at least three lens groups, such as, a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object. Upon zooming, at least two lens groups are made moveable. On the other hand, upon focusing at any focal length in the focal-length range defined by the short focal length extremity and the long focal length extremity, any one of the lens groups other than the first lens group is made moveable. Furthermore, at the shortest object-to-image distance, an image magnification of more than xc2xd is obtained at the long focal length extremity. The shortest object-to-image distance is defined as the shortest distance between the object and the image (image plane), which is inherent to a lens system under the condition that no accessories, such as a close-up lens element, an extension tube, bellows, etc., are used.
According to the above-explained macro zoom lens system, miniaturization and weight reduction can be achieved, since a lens group other than the first lens group is arranged to be a focusing lens group.
A four-lens-group macro zoom lens system can be obtained by providing positive fourth lens group, for example, on the image-side of the third lens group.
In both cases of the three-lens-group and four-lens-group macro zoom lens systems, focusing from infinity towards the shortest object-to-image distance can be performed by moving the second lens group.
In the case where the second lens group is the focusing lens group, the macro zoom lens system preferably satisfies the following condition:
0.3 less than XFT/(DZTxe2x88x92DZW) less than 10xe2x80x83xe2x80x83(1)
wherein
XFT designates the amount of advancing of the second lens group, at the long focal length extremity, from infinity towards the shortest object-to-image distance;
DZT designates the distance between the second principal point of the first lens group and the first principal point of the second lens group at the long focal length extremity when photographing an object at an infinite distance; and
DZW designates the distance between the second principal point of the first lens group and the first principal point of the second lens group at the short focal length extremity when photographing an object at an infinite distance.
The macro zoom lens system can also satisfies the following condition:
xe2x88x920.9 less than mT less than 0xe2x80x83xe2x80x83(2)
wherein
mT designates the magnification of the second lens group at the long focal length extremity when photographing an object at an infinite distance.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-254347 (filed on Sep. 8, 1999) which is expressly incorporated herein by reference in its entirety.